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Study on the Corrosion and Wear Characteristics of Magnesium Alloy

AZ91D in Simulated Body Fluids

R. Vaira Vignesh

Department of Mechanical Engineering,

Amrita School of Engineering, Coimbatore,

Amrita Vishwa Vidyapeetham, Amrita University, India.

R. Padmanaban*

Department of Mechanical Engineering,

Amrita School of Engineering, Coimbatore,

Amrita Vishwa Vidyapeetham, Amrita University, India.

*dr_padmanaban@cb.amrita.edu

M. Govindaraju

Department of Mechanical Engineering,

Amrita School of Engineering, Coimbatore,

Amrita Vishwa Vidyapeetham, Amrita University, India.

Manuscript Click here to access/download;Manuscript;Manuscript Revised 3Final.docx

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Study on the Corrosion and Wear Characteristics of Magnesium Alloy

AZ91D in Simulated Body Fluid

Abstract

Bioimplants made of metallic materials induce stress shielding effect and delayed

osteoblast activity during in-vivo experiments. Bioimplants also suffer corrosion, wear, and

combined effect of corrosion – wear during their service time. Bioimplants made of magnesium

alloys results in negligible stress shielding effect, owing to its similarity with bone’s elastic

modulus. However the soft matrix of magnesium alloy is susceptible to high wear rates. In this

study, magnesium alloy AZ91D is subjected to corrosion test (immersion and electrochemical),

adhesive wear and simultaneous corrosion – wear test to test the significance of the body fluid in

the corrosion – wear rate of the bioimplants. The surface morphology, elemental composition, and

phase composition of the specimens are characterized using FE-SEM, EDS, and XRD analytic

techniques. The results indicate that the simulated body fluid has significant effect on the corrosion

rate and corrosion – wear rate of the specimens.

Keywords: Magnesium alloy, Adhesive wear, Corrosion, Simulated Body Fluid, Radial Basis

Function

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1. Introduction

The biocompatible nature has enabled magnesium and magnesium alloys as one of the

candidature materials for bioimplants application [1, 2]. The first response of human immune

system to bioimplant is adhesion of protein to the surface of the bioimplants. This protein coating

corrodes the bioimplant material. Due to high chemical activity, bioimplants made of magnesium

alloys are prone to high corrosion rates with evolution of hydrogen gas [3, 4]. The rapid evolution

of hydrogen gas during corrosion of the bioimplant is a lethal phenomenon to the neighboring

tissues [5]. So the bioimplants should be engineered to withstand the effects of protein coating,

physiological pH, chloride ion from physiological fluid, change in human body temperature, and

have a controlled corrosion rate.

The first generation of biomedical bioimplants stressed on physical properties similar to

that of the replaced bone and minimized toxicity to the neighbor tissue [6, 7]. The second

generation of biomedical bioimplants emphasized on the ability of the biomaterials to (i) interact

with physiological environment (ii) enhance tissue bonding and osteoblast activity (iii) degrade

progressively with healing of tissues or bone. The current third generation of bioimplants is

designed to stimulate precise cellular response at the molecular level [8-13]. The material chosen

for biodegradable bioimplant should have gradual loss in mechanical strength and material by

corrosion, which should be counteracted by the newly forming bone [14, 15]. With mechanical

properties similar to that of bone, magnesium alloy AZ91D could be used for bioimplants.

However the corrosion rate of AZ91D is rapid in the chloride containing physiological

environment. The corrosion rate and hence the properties of AZ91D could be contrived using

various processing or coating techniques such as heat treatment, micro arc oxidation, vapor

deposition etc.

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Toshiyuki Nishio et al., [16] found that the microstructure of unidirectionally solidified

AZ91D alloy, consisted of α phase and nano sized β phase particles dispersion. The dispersion of

β phase increased tensile strength (TS) of processed alloy two folds. Mamoru Mabuchi et al., [17]

found that isotropic grain shape of directionally solidified AZ91D alloy enhanced the TS and proof

stress at 200 oC. Shanthi et al, [18] compared the wear behavior of recycled AZ91D alloy

(mechanical milling) and cast AZ91D alloy. The fine grained structure produced by mechanical

milling process did not increase the adhesive wear resistance, over a range of velocity at normal

load of 10N. Zafari et al., [19] found the critical temperature for transition of wear from mild to

severe in AZ91D alloy at various normal loads. Severe plastic deformation was observed at normal

load of 40 N and at testing temperature of 100oC. The formation of oxide layers at lower normal

load of 5N, increased the wear resistance at temperatures 100oC to 250oC. Severe softening of the

alloy at temperatures above 300oC, dramatically reduced the wear resistance of the alloy.

Zafari et al., [20] found that the formation of oxide layer decreased the wear rate of AZ91D

by 8% while the wear rate increased with increase in test temperature beyond 100oC. Softening of

β phase and formation of more slip systems, reduced the hardness and wear resistance. Chen and

Alpas [21] found that increase of contact temperature above 74°C, inducted severe wear in AZ91D

alloy during adhesive wear test. The wear rate and contact temperature increased with sliding

distance during dry sliding wear tests.

Witte et al., [22] found that AZ91D has good biocompatibility and corrosion resistance in

in-vivo studies. Choudhary et al., [23] found that AZ91D alloy is susceptible to stress corrosion

cracking in simulated body fluid (SBF) solution in the regime of low strain rate. They observed

transgranular cracks in the fractured specimens, though intergranular SCC and combined SCC was

prevalent in AZ91D alloy. Xue et al., [24] found that the in vivo corrosion rate of AZ91D alloy

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was lower than the in-vitro corrosion rate (SBF at 37°C). No biocompatibility issues were observed

for implanation period of two months. Walter et al., [25] investigated the effect of surface

roughness on the polarization resistance of AZ91D alloy tested in SBF and found that the alloy

with rough surface had lower polarization resistance than smooth surfaced alloy. The severity of

localized degradation of was higher for the specimen with rough surface. Tahmasebifar et al., [26]

studied the effect of pressure molding process variables on the cell adhesion and proliferation

properties of AZ91D alloy. The plates with high porosity and rough texture increased the cell

adhesion and proliferation. Wen et al., [27] studied the bio-degradability and surface chemistry of

AZ31D and AZ91D alloys subjected to immersion and electrochemical tests in modified SBF.

With increase in immersion period from 1 day to 24 days, the corrosion rate of AZ91D was

significantly lower than that of AZ31D.

The mechanical and tribological behavior of alloys depends on the corrosivity of the

environment, in which the alloys deliver their functionality. So a study to investigate the effect of

corrosive physiological environment on the wear rate of AZ91D alloy becomes obligatory. In this

study, an attempt has been made to analyze the effect of corrosive SBF environment (Dulbecco’s

phosphate buffered solution) on the wear rate of the AZ91D specimens. The effect of wear process

parameters on the adhesive wear rate and corrosion – wear rate was also investigated. A hybrid

model (polynomial function and radial basis function) was developed to investigate the effect of

adhesive wear parameters of process on the wear rate of specimens. The corrosion rate of AZ91D

was evaluated by conducting immersion test and electrochemical corrosion test. The influence of

corrosion products on the corrosion mechanism of AZ91D in SBF was also investigated.

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2. Materials and Methods

2.1 Base material

Rolled AZ91D plate of thickness 4 mm was used in the study. Chemical composition was

analyzed using optical emission spectroscopy and the composition is given in the Table 1.

Table 1. Composition of as cast AZ91D alloy

Element Al Zn Si Mn Fe Cu Ni Mg

Composition (wt. %) 8.72 0.51 0.04 0.31 0.001 0.002 0.01 Balance

2.2 Base material characterization

2.2a Microstructure

Specimens of dimension 10 mm x 10 mm were cut from the base material and mounted

using cold setting compound. The surface of the specimens were prepared as outlined by the

standard ASTM E3 – 11. The surface of the specimen was prepared using automatic disc grinder

using different grades of abrasive sheet, starting from coarse grade abrasive sheet to very-fine

grade abrasive sheet. A mirror polish was obtained in the specimen by polishing the specimen in

diamond slurry, which was followed by polishing in alumina suspension against soft velvet cloth.

That was followed by The etchant was prepared by mixing 100 ml of 95% ethanol, 6 g picric acid,

5 ml acetic acid and 10 ml distilled water, as prescribed ASTM E407 – 07. The polished surface

of the specimens was etched for 10 seconds by a gentle swab using cotton dipped in etchant, which

was in turn followed by a cold water rinse. The microstructure was observed in an optical

microscope (Make: Carl Zeiss, Model: Axiovert 25).

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2.2b Microhardness test

As described in the microstructure section, the specimens were prepared for microhardness

test (unetched). Vicker’s microhardness measurements were made using a microhardness tester

(Make: Mitutoyo, Model: MVKH1) as outlined by the standard IS 1501:2002. The diamond

indentor was plunged into the specimen under an axial load of 9.80 N and the indentor was

withdrawn after 15 s.

2.2c Tensile test

The specimens for tensile test were prepared and tested in a universal testing machine

(Make: Tinius Olsen, Model: H25KT) as per the guidelines of the standard ASTM B557 – 10. The

dimensions of the tensile test specimens are as follows: gauge length: 50.8 mm, reduced section

width: 12.7 mm, reduced section length: 57.2 mm, width of grip section: 19.0 mm, length of grip

section: 50.8 mm. The stress – strain graph was plotted by increasing the strain (incremental value

of 0.01) and obtaining the force correspondingly. The yield strength was determined by offset

method (0.2%).

2.3 Corrosion and Wear tests

2.3a Immersion corrosion test

The immersion test was performed to assess the change in pH of the SBF and corrosion

rate of the base material with increase in time. The SBF used for the immersion test was Dulbecco’s

Phosphate Buffered solution. The specimens for immersion test were prepared and tested in SBF

as per the guidelines of the standard ISO 10993 – 15. The composition of the SBF solution is given

in the Table 2. The temperature of the solution was maintained at 37±0.1°C using a temperature

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controlled water bath and the pH of the SBF solution was measured using a pH meter with a

sensitivity of ±0.01.

Table 2. Composition of Dulbecco’s phosphate buffered solution

Sl. Component name Chemical formula Concentration

(mg / L)

1 Anhydrous Calcium Chloride CaCl2 100

2 Anhydrous Magnesium Chloride MgCl2 47

3 Potassium Chloride KCl 200

4 Potassium phosphate monobasic KH2PO4 200

5 Sodium Chloride NaCl 8000

6 Anhydrous Sodium Phosphate dibasic Na2HPO4 1150

2.3b Electrochemical Corrosion test

The specimens for electrochemical corrosion test were cut from the base material to the

required dimension of 60 mm x 10 mm. The surface of the specimens were prepared as per the

procedure outlined in the section Microstructure and degreased with acetone. The prepared

specimens were placed in a desiccator filled with calcium chloride (CaCl2) to avoid the ill effects

of moisture and air. The specimens were masked using an insulation tape and an area of 1 mm2

was left unmasked for exposure to SBF solution, which is Dulbecco’s phosphate buffered solution.

The electrolyte solution was prepared by mixing the essential amount of salts in distilled water as

listed in Table 2. The specimen (working electrode), platinum wire (counter electrode), and

standard calomel electrode (reference electrode) constituted the electrochemical cell setup. The

electrochemical cell setup was immersed in a water bath, which was maintained at a physiological

temperature of 37±0.1°C and a physiological pH of 7.0.

Potentiodynamic polarization is one of the techniques in electrochemical corrosion test

used to deduce the corrosion current density, from the net current of anodic and cathodic

polarization by polarizing the test specimen in the electrolyte. Electrochemical workstation (Make:

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CH Instruments, Model: 680 Amp Booster) was used to perform potentiodynamic polarization

test. The potentiodynamic polarization test was performed in line with the standard ASTM G5-94.

Open circuit potential was established for the specimen for an immersion period of 120 minutes

and that potential was chosen as reference potential for conducting the potentiodynamic

polarization tests. The specimens were polarized at a scan rate of 5 mV.s-1 in the potential range of

-2.5 V to +2 V. The current sensitivity was chosen as 10-1 μA for the ECC tests.

2.4 Adhesive wear test

AZ91D samples of dimension 10 m x 10 mm were mounted in a steel hollow tube of outer

diameter 12 mm, inner diameter of 10 mm and height of 50mm, using cold setting compound to

prepare adhesive wear test specimens. The specimens were cleaned with acetone and weighed

using a precision weighing balance with a readability of 0.0001 g. The wear test was conducted at

par with the guidelines of the standard ASTM G99 – 95a in a pin on disc wear tester (Make: Ducom,

Model: TR20LE). The wear test parameters chosen for the study were load at the wearing contact

(load), relative sliding speed between the contacting surfaces (velocity) and accumulated sliding

distance (distance).

2.4a Statistical modelling

Face centered central composite design was used to develop the design matrix and generate

a mathematical model to interrelate the wear test parameters with the wear rate of the specimens.

The chosen level of process parameters and the design matrix for experimental trials for adhesive

wear test is given in Table IV.

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The regression models developed by response surface designs could not map the response

variables that have complex non-linearity characteristics with the input process parameters [28].

A hybrid model integrating the polynomial function and radial basis function was developed to

study the effect of wear process parameters on the adhesive wear rate of the specimens. Radial

Basis Function (RBF) is a special class of function and it is typically of Gaussian type [29]. The

advantage of multiquadratic RBF over Gaussian type is that it delivers finite global response and

it is given by the equation (1)[30].

f(𝑥) =√𝑟2 + (𝑥 − 𝑐)2

𝑟 (1)

Multiquadratic RBF was chosen to build the model for the response variable (adhesive wear rate).

The characteristic equation for function approximation using RBF network is given by equation

(3).

𝑅𝐵𝐹 = 𝑓(𝑥) = ∑ 𝑤𝑖 ∗ ∅(||𝑥 − 𝑥𝑖||)

𝑛

𝑖=1

(2)

Where 𝑓(𝑥) is the approximation function, 𝑛 is the number of RBFs, 𝑤𝑖’s are the weights. The

characteristic polynomial function– RBF mathematical model is given by equation (4).

y(𝑥) = PF + RBF (3)

Where 𝑦 is the response variable, 𝑃𝐹 is polynomial function and 𝑅𝐵𝐹 is radial basis function. If

the polynomial function is of degree 1, a linear – RBF is obtained.

2.5 Corrosion – wear test

A new approach was established to study the combined effect of corrosion and wear rate

of the specimens. The SBF is heated to 37±0.1°C using a temperature controlled water bath. The

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flow of the SBF was controlled using a control valve. The specimens were clamped in the adhesive

wear tester and the setup was confined using an enclosure. The wear test was performed in

accordance with the standard ASTM G99 – 95a with the flow of SBF (corrosive liquid). The

corrosion – wear test parameters were chosen similar to that of adhesive wear test parameters to

explicitly study the effect of SBF fluid on the corrosive wear rate of the specimens.

2.6 Metallurgical characterizations post corrosion and wear tests

The surface morphologies of the specimens subjected to tensile test, immersion test,

electrochemical corrosion test, adhesive wear test and corrosion wear test were observed using a

field emission scanning electron microscope (Make: Zeiss Sigma). The images were obtained at

electron acceleration voltage of 5kV and 10kV at various levels of magnification. The elemental

composition and the elemental map of the corroded specimens was determined using energy

dispersive x-ray spectroscopy (Make: Bruker). The constitutional phases of the base material,

adhesive wear test debris and corrosion – wear test debris were analyzed using X – ray

diffractometer (Make: Rigaku, Model: Ultima 4). XRD results of the specimens were obtained at

continuous scanning mode using Copper Kα radiation at a scan rate of 2° per minute.

3. Results and Discussion

3.1 Base material characterization

3.1a Microstructure

The optical micrograph of the base material at 200x magnification displaying well-

developed α – Mg (α phase) dendrites with secondary arms is shown in the Figure 1. Beyond the

solubility limit of Al in Mg, a brittle intermetallic secondary phase β – Mg17Al12 (β phase)

precipitated out from the supersaturated primary phase α phase of the alloy. The phases present in

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the base material are identified as α phase, β phase and (α + β) eutectic phase. Picric acid in the

etchant darkened α phase and so α phase / β phase was observed as dark region / bright region in

the micrograph respectively. The coarse branched structure observed at the grain boundaries and

inter dendritic region (gray region) was identified as eutectic phase. XRD analysis was performed

to investigate the existing phases present in the base material AZ91D. As shown in the Figure S1,

distinct diffraction peaks were observed for α phase and β phase of the alloy AZ91D. Owing to

the lesser concentration of Zn in the matrix, diffraction peaks of Mg – Zn phases were not detected.

3.1b Microhardness

Vicker’s microhardness was measured along a horizontal line in the specimen at 10

instances that were located at a distance of 1 mm from the previous instance (indentation). The

average microhardness of the base material was found to be 57.9±3.15 Hv. The standard error of

the measurements was 0.996438.

3.1c Tensile test

The stress – strain graphs of three tensile test specimens are shown in the Figure S2. The

test specimens exhibited an average yield strength (offset = 0.2%) of 135 MPa. As shown in the

Figure 2, large porosities and oxide layers were observed in the FE-SEM fractography of the

fractured surface. The fractured surface with dimples as primary feature indicated ductile fracture

[31, 32]. The β phase network at the grain boundaries tends to crack or debond from the matrix

under lower stress during tensile deformation [33]. Therefore brittle fracture was observed at some

instances on the fractured surface.

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3.2 Corrosion and Wear test

3.2a Immersion test

The concentration of the Hydrogen ions (H+) in the SBF was measured using a pH meter.

It is observed from the Figure S3 (a) that the concentration of H+ in SBF decreases with increase

in immersion period. This indicated the evolution of H2 gas as a result of corrosion phenomenon

on the surface of the specimens. The corrosion products on the surface of the specimens immersed

in SBF were cleaned using chromic acid and then the mass loss was calculated. The rate of

variation of mass decreased and became stable with increase in immersion time. The corrosion

rate of the specimens immersed in SBF for different intervals of time is shown in the Figure S3

(b). The corrosion rate of the specimen after 24 hours of immersion was 4.37 mm.year-1. The

corrosion rate steadily decreased and stabilized at an average corrosion rate of 1.05 mm.year-1 after

144 hours of immersion. The corrosion rate of the specimens decreased by a factor of 50%, 29%,

22%, 10%, 0.23% and 0.02% in the proceeding days of immersion.

The surface morphology of the specimens was observed using FE-SEM after immersion in

SBF for different time lengths. The surface of the specimens IC1, IC3, IC5 and IC7 is shown in

the Figure 3 (a), Figure 3 (b), Figure 3 (c) and Figure 3 (d) respectively. The surface was covered

by a corrosion layer, with many shallow cracks and micro pores on its surface. During initial hours

of immersion, the formation of Mg(OH)2 layer acted as barrier to prevent the matrix from

absorbing Ca and P from the SBF. Mg(OH)2 layer is vulnerable in the presence of chloride ions

(Cl-) [34]. Presence of Cl- ions in SBF and layered structure of Mg(OH)2, cracked the continuous

corrosion layer of Mg(OH)2 [15]. Flower like clusters bloomed over the surface of the specimen

after an immersion period of 24 hours, as shown in the Figure 3 (a). As observed from the Figure

3 (d), the flower like structures fell off from the surface, with increase in immersion time. The

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deposition of the corrosion products suppressed the corrosion rate of the specimens and reached a

stable state after 144 hours of immersion. The flower like corrosion products could be magnesium

carbonate, sulphate or phosphate.

Table 3. EDS analysis of the specimens corroded in immersion test

Sl. Element

Normalized concentration

Wt. % Atomic concentration Wt. %

Error (3 σ)

Wt. %

IC1 IC7 IC1 IC7 IC1 IC7

1 Aluminium 0.16 0.11 0.13 0.09 0.10 0.10

2 Magnesium 12.32 15.97 11.17 14.59 1.42 1.88

3 Sodium 1.40 1.93 1.34 1.86 0.28 0.36

4 Potassium 0.07 0.05 0.04 0.03 0.09 0.09

5 Calcium 21.98 18.74 12.08 10.38 0.12 1.22

6 Chlorine 0.46 0.60 0.29 0.38 1.35 0.13

7 Oxygen 44.61 41.41 61.43 57.48 11.55 11.22

8 Phosphorous 19.01 21.19 13.52 15.19 1.53 1.77

Total 100 100 100 100 - -

EDS analysis was performed on the corroded specimen to quantify the elements present in

the corrosion products. The result of the EDS analysis of the corroded specimens after 24 hours

(IC1) and 168 hours (IC7) of immersion are given in the Table 3. The peaks indicated the presence

of Mg, Na, Ca, P, Cl, O elements in the corrosion products. During initial period of immersion,

increase in Ca/P ratio increased the corrosion rate of the specimens. However, the Ca/P ratio

decreased with increase in immersion time, reducing the corrosion rate. The reduction in Ca on

the surface of corroded specimen favored the formation of hydroxy apatite layer

(Ca10(PO4)6(OH)2) or calcium magnesium phosphate layer (Ca10-xMgx(PO4)6(OH)2) [8-10, 13].

Previous studies proved that the formation of hydroxyapatite layer on the bioimplant material

support the bone growth and bioimplant – bone integration [22, 35]. Elemental mapping of Mg,

Al, Ca, P and O in the corroded area of specimen IC1 and IC7 are shown in the Figure 4 (a) and

Figure 4 (b) respectively. The elemental mapping revealed that the corrosion products increased

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with increase in immersion time. The high concentration of Ca and P in the corrosion layer of the

specimen IC7 is attributed to the Ca – P salt containing Mg and / or Mg(OH)2.

XRD analysis of the corroded specimen IC7 is shown in the Figure S4. Prominent peaks

were detected for α phase and β phase. Less intense peaks of MgO and Mg(OH)2 were also detected

in XRD. Besides many peaks present in the 2θ range between 20° and 35° indicated the presence

of Ca10(PO4)6(OH)2 or Ca10-xMgx(PO4)6(OH)2 on the corrosion layer.

The corrosion mechanism of the AZ91D is significantly influenced by microstructural

phases in the alloy matrix and the composition of the SBF solution. As shown in the Figure 5 (a),

many circle like patches were observed on the FE-SEM image of the specimen IC7 after removal

of corrosion products using chromic acid. At higher magnification, protruded structures with

shallow pits were observed. When the specimen IC7 was immersed in chromic acid, low

dissolution of β the phase resulted in protrusions while high dissolution of α phase resulted in

shallow pits. It was observed that the dissolution propensity of α phase, left relatively stable grain

boundaries of β phase. This attests that the β phase was homogeneously distributed in the matrix

and acts as barrier to corrosion.

3.2b Electrochemical Corrosion test

After establishing a stable open circuit potential, the specimen was potentiodynamically

polarized from -2.5 V to +2.0V at a scan rate of 5mVs-1. The Tafel plot of the tested specimen is

shown in the Figure S5. The electrochemical parameters extracted from the Tafel plot are as

follows: Corrosion current, icorr = 1.943x10-6 A; Corrosion potential, Ecorr = -1.448 V. Ecorr

determines the thermodynamic tendency towards corrosion while icorr indicated the velocity of

uniform corrosion. The corrosion current was converted to corrosion rate using the equation (4).

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Corrosion rate (𝑚𝑚 𝑦𝑒𝑎𝑟⁄ ) =𝑘 × 𝑖𝑐𝑜𝑟𝑟 × 𝐸′

𝐴 × 𝜌 (4)

Where k is a constant (3272), E’ is equivalent weight in grams/equivalent (12.1525

grams/equivalent), A is exposure area (0.01 cm2) in cm2 and ρ is density (1.74 grams.cm-3) of the

specimen in grams.cm-3. The corrosion rate calculated from the Tafel region was 4.44 mm.year-1.

The corrosion rate calculated from the mass loss of the specimen was found to be lower than the

corrosion rate calculated from the Tafel region. The surface morphology of the corrosion specimen

is shown in the Figure 5 (b). The presence of micro pores and cavities on the surface of the

corroded specimen indicated high corrosion rate of the specimen in potentiodynamic polarization

test.

During the initial immersion period of the specimen, Mg in the alloy matrix dissolves and

forms a corrosion layer of Mg(OH)2. The equations describing the reaction on the surface of the

specimen are given by equation (4) to equation (6). The corrosion layer Mg(OH)2 was stirred by

the evolution of H2 gas, which promotes further corrosion of the specimen.

Mg → 𝑀𝑔2+ + 2𝑒− (5)

2𝐻2O + 2𝑒− → 𝐻2 ↑ +2𝑂𝐻− (6)

𝑀𝑔2+ + 2𝑂𝐻− → 𝑀𝑔(𝑂𝐻)2 (7)

Mg(OH)2 is relatively more active than Mg and hence they could be regarded as the defect

sites that are ideally suited for corrosion in Cl- ion rich environments. With increase in immersion

time, anodic reaction results in more dissolution of Mg2+ ions and more consumption of phosphate

and carbonate ions from the SBF solution. This increased the Cl- ion concentration in the SBF

solution. Hence, the Cl- ions are preferentially adsorbed on the specimen surface. Owing to its

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small radius, Cl- ions penetrate the corrosion layer and converts Mg(OH)2 to MgCl2. The formation

of MgCl2 from Mg(OH)2 is summarized in the equation (8).

𝑀𝑔(𝑂𝐻)2 + 2𝐶𝑙− → 𝑀𝑔𝐶𝑙2 + 2(𝑂𝐻)− (8)

The high solubility of MgCl2 in the electrolyte creates many exposed sites on the surface,

which enables more accessibility of Mg2+ ions to phosphate ions (HPO42- or PO4

3-), and Ca2+ ions

in the SBF solution. This is turn resulted in the formation Ca10-xMgx(PO4)6(OH)2 or

Ca10(PO4)6(OH)2, in accordance with the published research [27, 36]. The formation of such

compact and insoluble phosphates retarded dissolution of Mg from the surface of the specimen.

Hence the corrosion rate of the specimens decreased with increase in immersion time, which is

evident from the immersion test results given in the section 3.2a Immersion test. This layer reduced

the permeability of Ca2+ and PO43- ions into the interface between the corrosion layer and the

matrix. Hence the corrosion layer consisted of more Ca, P and O than other elements, which is in

agreement with the result of EDS element map. As evident from literature, the difference in

corrosion potential between α phase and β phase is approximately 1 V. The α phase is more anodic

with respect to the β phase, which induces micro galvanic corrosion in the matrix.

3.3 Adhesive wear

The adhesive wear test was performed by varying the wear test parameters namely normal

load, sliding velocity, sliding distance at three levels as per face centered central composite design.

Equation (9) was used to calculate the wear rate of the specimens and the calculated wear rates are

given in the Table 4.

𝑊𝑒𝑎𝑟 𝑟𝑎𝑡𝑒 (𝑔 𝑁𝑚⁄ ) =∆𝑚

𝐹 × 𝐿 (9)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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The wear rate of the specimen AW1 tested at a load of 9.8 N, a sliding velocity of 1 ms-1

and a sliding distance of 600 m was found to be 0.3911 g / Nm, which was the least among the

tested specimens. The metallographic examination of the worn out surface shows loose wear debris

as seen in the Figure 6 (a). The delamination of surface oxide layer or subsurface material layer

resulted in the formation of loose wear debris.

Table 4. Design matrix and experimental results of adhesive wear test

Sl. Coded Real Wear Rate (10-6)

(g / N. m) Load Velocity Distance Load (N) Velocity (m/s) Distance (m)

AW1 -1 -1 -1 9.8 1 600 0.3911

AW2 1 -1 -1 29.4 1 600 0.4535

AW3 -1 1 -1 9.8 3 600 3.4694

AW4 1 1 -1 29.4 3 600 1.9785

AW5 -1 -1 1 9.8 1 1200 2.8316

AW6 1 -1 1 29.4 1 1200 2.0493

AW7 -1 1 1 9.8 3 1200 12.3300

AW8 1 1 1 29.4 3 1200 6.7857

AW9 0 0 0 19.6 2 900 2.3413

AW10 0 0 0 19.6 2 900 2.5680

AW11 0 0 0 19.6 2 900 2.3243

AW12 0 0 0 19.6 2 900 2.4603

AW13 -1 0 0 9.8 2 900 3.4240

AW14 1 0 0 29.4 2 900 1.1451

AW15 0 -1 0 19.6 1 900 2.7381

AW16 0 1 0 19.6 3 1000 1.6480

AW17 0 0 -1 19.6 2 600 2.3129

AW18 0 0 1 19.6 2 1200 2.2747

AW19 0 0 0 19.6 2 900 2.4943

AW20 0 0 0 19.6 2 900 2.3929

The wear rate was high (12.33 g / Nm) for the specimen AW7 tested at a load of 9.8 N, a

sliding velocity of 3 ms-1 and a sliding distance of 1200 m. The surface morphology of the worn

out specimen AW7 displayed a severe wear regime, which was characterized with massive surface

deformation and large amount of wear debris, as shown in the Figure 6 (b). During wear test, the

material layer adjacent to the contact surface had suffered an extensive plastic deformation. These

plastically deformed layers extruded out of material surface, which was subsequently removed as

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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flat wear debris particles. Hence the worn out surface had deep grooves indicating severe wear of

the specimens. XRD analysis of the worn out debris is shown in Figure S6. The XRD peaks

confirmed the presence of α phase, β phase and MgO, which indicated that the delamination of

surface oxide layer and subsurface material layer during wear process. The broadening of the peaks

indicated the plastic deformation of the material during the adhesive wear test.

A hybrid model composting of polynomial function and radial basis function was

developed using the experimental data to investigate the influence of wear test parameters on the

wear mechanism and wear rate of the specimens. The model is given by the equation (10) and was

developed with the coded level of the wear test parameters.

Wear = 1 + 2 × Distance + 6 × Load + 9 × Velocity + 4 × Distance

× Load + 5 × Distance × Velocity + 8 × Load × Velocity

+ RBF

(10)

The RBF network was developed using a multiquadratic kernel with 0 center, a global

width of 2 and regularization parameter of 0.0001. The root mean squared error (RMSE) and

coefficient of determination (R2) are the critical statistical factors that determine the prediction

efficiency of the developed model. R2 value of the model was calculated to be 0.97 and RMSE

value of the predicted value was found to be 0.03. Closeness of R2 value to one and closeness of

RMSE value to zero indicated the high prediction efficiency of the developed model.

The load at which the wear of the specimen changes either from mild wear to severe wear

or from severe wear to mild wear is known as transition load. The effect of load on the wear rate

of the specimens at various sliding velocities for a sliding distance of 600 m is shown in the Figure

7 (a). The specimens tested exhibited a transition load of 19.6 N at all sliding velocities. The wear

phenomenon changed from severe wear to mild wear at sliding velocities of 1 ms-1 and 2 ms-1 and

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

20

changed from mild wear to severe wear at a sliding velocity of 3 ms-1. The effect of load on the

wear rate of the specimens at various sliding velocities for a sliding distance of 900 m is shown in

the Figure 7 (b). The specimens tested exhibited a transition loads of 19.6 N and 23 N at sliding

velocities of 1 ms-1 and 3ms-1 respectively. The wear phenomenon changed from severe to mild

wear at 1 ms-1 and changed from mild to severe wear at a sliding velocity of 3ms-1. The wear rate

decreased with increase in load at sliding velocity of 2 ms-1. The effect of load on the wear rate of

the specimens at various sliding velocities for a sliding distance of 1200 m is shown in the Figure

7 (c). The transition load for change in wear phenomenon from severe to mild wear for the

specimens tested was 19.6 N at sliding velocities of 2 ms-1 and 3 ms-1. However no specific change

in wear phenomenon was observed for the specimens tested at a sliding velocity of 1 ms-1.

The effect of sliding distance and sliding velocity on the wear rate of the specimens is

shown in the Figure 8 (a). At low load of 9.8 N, the wear rate increased with increase in sliding

distance. Under mild wear regime, the wear mechanism transforms from oxidational wear to

delamination wear. However, at loads greater than 19.6 N, the wear rate decreased with more

sliding distance. The increase in wear resistance of the specimens slid for more distance at higher

loads indicated the formation of work hardened matrix layer. The effect of sliding distance and

sliding velocity on the wear rate of the specimens is shown in the Figure 8 (b). It is observed that

the wear rate decreased with increase in sliding velocity. The work hardening of specimens with

more slid distance decreased their wear rate. The softness of the material matrix resulted in high

wear rate for the specimens that slid less than 900 m, at slow sliding velocities of 1ms-1 and 2 ms-

1. Figure 8 (c) shows the effect of applied load and sliding velocity on the wear rate of the

specimens. It is observed that the wear rate was high for the specimen tested at low load and high

sliding velocity and low for the specimen tested at high load and high sliding velocity.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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3.4 Corrosion – Wear

The corrosion – wear tests were conducted in the SBF environment, adopting wear test

parameter combinations that resulted in the most and the least adhesive wear rate. The chosen level

of wear test parameters and the corresponding corrosion – wear rate is given in the Table 5. It is

observed that the corrosion – wear of the specimens was influenced by the SBF solution. The

surface morphology of the specimen CW1 and CW2 is shown in the Figure 9 (a) and Figure 9

(b) respectively. It is observed that the surface of the specimens CW1 and CW2 was devoid of

wear tracks and the friction coefficient was lower for the specimens subjected to corrosion – wear

test than the specimens subjected to adhesive wear test.

Table 5. Experimental results of corrosion – wear test

Sl.

Coded Real Corrosion Wear

Rate (10-6)

(g / N. m)

Load

(N)

Velocity

(m/s)

Distance

(m)

Load

(N)

Velocity

(m/s)

Distance

(m)

CW1 -1 -1 -1 9.8 1 600 2.17

CW2 -1 1 1 9.8 3 1200 2.41

The interaction of SBF with the specimen produced debris and striations on the surface.

The striations were higher in the specimen CW2 than the specimen CW1. Correspondingly, the

corrosion – wear rate was higher in the specimen CW2 than the specimen CW1. However the

corrosion – wear rate of specimen CW1 was higher than the specimen AW1, which had undergone

adhesive wear. The lubricating action of the SBF solution reduced the friction coefficient between

the specimen and the counter disc as well as the heat generated at the specimen – counter disc

interface. This resulted in less corrosion – wear rate of the specimen CW2.

4. Conclusions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Magnesium alloy AZ91D was characterized for microstructure, microhardness and tensile

strength. The base material was subjected to corrosion test, wear test and combined corrosion –

wear test. The conclusions of the study are as follows.

Microstructure of magnesium alloy AZ91D consists of α – Mg, β – Mg17Al12 and (α+β)

eutectic phases. The β phase is distributed as coarse continuous precipitates in the matrix.

Magnesium alloy AZ91D favors formation of hydroxy apatite layer on its surface with

increase in immersion time in body fluids. The corrosion potential and the corrosion rate

interpolated from the Tafel region is -1.448 V and 4.44 mm.year-1 respectively.

Statistical modelling of the adhesive wear test elucidates the transition load with respect to

change in wear test parameters. SBF has significant influence on the wear test of the

specimens, as observed from the corrosion – wear test.

Acknowledgement

The authors are grateful to Amrita Vishwa Vidyapeetham, India for their financial support

to carry out this investigation through an internally funded research project no. AMRITA/IFRP-

20/2016-2017.

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List of Figures

Figure 1. Microstructure of AZ91D depicting the α phase (dark region), β phase (bright region)

and (α+β) eutectic phase (grey region)

Figure S1. XRD analysis of AZ91D showing the prominent peaks for α phase and β phase in the

base material AZ91D

Figure S2. Stress – Strain graph of AZ91D

Figure 2. Fractography of the tensile test specimen at magnification of 4000 X showing dimples

and ruptured surface indicating a combined ductile and brittle fracture mechanism

Figure S3. a) Increase in pH value of SBF indicates the evolution of H2 gas with increase in

immersionn time; b) Decrease in corrosion rate of AZ91D indicates the formation of corrosion

resistant layer on the surface with increase in immersion time

Figure 3. Surface morphology of the specimen a) IC1 after 24 hours of immersion showing flower

like clusters; b) IC3 after 72 hours of immersion with diminished flower like clusters; c) IC5 after

120 hours of immersion showing wide cracks and a few flower like clusters; d) IC7 after 168 hours

of immersion showing a few flower like clusters and a thin layer of corrosion products on the

surface

Figure 4. Elemental mapping of the specimen a) IC1 after 24 hours of immersion; b) IC1 after 168

hours of immersion indicating the high deposition of Ca and P on the surface

Figure S4. XRD analysis of the corrosion products of specimen IC7 after 168 hours of immersion

showing the prominent peaks for α phase, β phase, MgO and Hydroxyapatite

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

25

Figure 5. a) Surface of the specimen IC7 after cleaning the corrosion products showing protrusion

(β phase) and shallow pits (α phase, which was dissolved by chromic acid); b)Surface morphology

of AZ91D after potentiodynamic polarization test showing micropores at magnification of 3000X

Figure S5. Potentiodynamic polarization plot of AZ91D in SBF solution at 37±0.1°C

Figure 6. FE-SEM image of the specimen after adhesive wear test a) Specimen AW1 exhibiting

mild wear; b) Specimen AW7 exhibiting severe wear

Figure S6. XRD analysis of specimen AW7 indicatign the presence of oxide layer along with the

microcontiuents of the alloy AZ91D

Figure 7. Effect of axial load and sliding velocity on the adhesive wear rate of specimens showing

transition load at a sliding distance of a) 600 m; b) 900 m; c) 1200 m

Figure 8. Effect of wear test parameters a) Distance and Velocity; b) Distance and Load; c) Load

and Velocity on the wear rate of AZ91D

Figure 9. FE-SEM image of the specimen a) CW1; b) CW2 after corrosion – wear test

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